This invention generally relates to the blow molding of plastic articles. More particularly, the invention relates to an improved extrusion head for producing a plastic parison that is used in blow molding plastic articles having non-circular cross-sections. The resulting parison has a varied distribution of the plastic material about the parison side wall.
Blow molding is well known technique where a hollow plastic parison is clamped within a mold and then blown or expanded outward into contact with the molding surfaces to form an article of the desired shape. The parison itself is formed by an extrusion process and numerous examples of blow molded articles can be cited for illustrated purposes. However, the most well known and easily recognized blow molded articles are probably, the, various types of liquid and beverage containers. Several specific examples include liquid detergent containers, one gallon milk jugs and motor oil containers. These examples are most often formed from high density polyethylene (HDPE) but other polymer materials are also used.
The typical extruded parison has an annular cross-section and a substantially constant wall thickness. This is usually good for forming containers having generally annular or circular cross-sections. However, when an article having a non-circular cross-section is blow molded from such a parison, those portions of the parison wall which subsequently form the corners of the article are subjected to the maximum elongation, expansion and stretch experienced by the parison. These portions are therefore thinner and weaker than the remaining sections of the article, even though their shape necessitates greater strength.
To ensure that these corners nave a sufficient wall thickness, the parison can be extruded with an increased wall thickness. Obviously, in and of itself, this will result in the remaining portions of the article also having an increased thickness. The resulting article therefore consumes more material than is actually needed, is heavier than desired and more costly to produce.
In attempting to solve the problems of side wall thickness variances and excessive material usage, parisons have been extruded with non-circular cross-sectional shapes, elliptical for example. Parisons have also been extruded with non-uniform wall thicknesses. A problem associated with the latter is that, in order to produce non-uniform wall thicknesses, non-uniform rates of extrusion are introduced into the molten plastic material. As the material exits the extrusion die head, it is often seen that the different extrusion rates cause the parison to close, collapse, curtain, wrinkle or twist. These are not just the result of varying material flow rates, but also a result of the inherent "bounce-back" or parison "swell" exhibited during discharge of the molten plastic. Before discussing parison swell in more detail, it is believed that a brief discussion of polymer flow characteristics would be of benefit.
Polymer materials flow in a viscous, an elastic or a visco-elastic fashion. Which particular flow depends on the nature of the polymer, polymer temperature, the degree of applied force, and the period of time over which the flow is maintained. Melted polymer materials under typical molding process conditions usually exhibit the characteristics of viscous flow and elastic flow simultaneously, that is, visco-elastic flow.
In viscous flow the polymer material continues and is deformed as a force is applied to it. When the force is removed the deformation remains. The rate of deformation is proportional to the force.
During elastic flow the polymer material deforms into a new shape and holds that shape as long as the force is applied. When the force is removed, the material returns to its original shape. This deformation is proportional to the force.
The viscous component of visco-elastic flow predominates in the processing of most melted polymer materials. However, the elastic component plays an important role and is the cause of the parison swell created during the discharge of the parison from the extrusion head. Process and polymer conditions which influence swell include, but are not limited to, higher extrusion pressures, lower melt temperatures, shorter tooling lands, higher polymer molecular weights, broader polymer molecular weight distributions, and larger extrusion internal head diameters relative to the die and mandrel diameters. All of the conditions tend to increase parison swell.
Parison swell occurs in two areas of the parison. The first is what the industry refers to as diameter swell. In diameter swell, the diameter of the resulting parison is larger than the orifice diameter created by the die and mandrel. The second is generally referred to as weight swell. With weight swell, the wall thickness of the parison is thicker than the orifice gap created by the die and mandrel.
Time is an important consideration in the amount of parison swell. A polymer will respond with a mostly elastic reaction when the force is applied quickly for a very short period of time. On the other hand, a polymer will respond with a mostly viscous reaction when the force is applied slowly for an extended period of time.
During discharge, the polymer material is in the die/mandrel gap area for a very short period of time. Moments earlier, up-stream in the extrusion head, this polymer held some other shape for a relatively longer period of time. This earlier shape is always thicker and usually larger in diameter than the die/mandrel orifice. Once the polymer discharges from the die/mandrel orifice, the elastic flow component attempts to return the material partially back to the earlier larger shape. In other words parison swell occurs.
If the die/mandrel lands are long the material will begin to assume the new shape of the die/mandrel orifice and gap. The viscous flow component becomes more predominant and the parison swell is reduced. Long narrow die/mandrel gap also creates a restriction that retards the parison flow rate.
One way to extrude a thicker section into a specific area of the parison wall is shape the surfaces of the die or mandrel (or both) of the extrusion head. This is generally referred to as die shaping. Die shaping consists of widening the die orifice or die gap in that area where the resulting thicker portion of the parison will correspond to the appropriate section of the article in need of a thicker wall portion. A portion of the die orifice is widened by machining the appropriate thickness out of the land of either the die ring or mandrel while running out or smoothly decreasing the depth of this cut in a circumferential progression away from the maximum gap portion.
One type of die shaping, know as parallel land die shaping, is a very effective way of changing the thickness of the parison by enlarging the die/mandrel gap in selected areas. During parallel land die shaping, a portion of the die is shaped at the same angle as the original land and over the entire axial land length. The parison flow rate through the die/mandrel gap and the parison swell is greatest where the die shaping is positioned and retarded where the die shaping is not positioned. These different parison flow rates and swells create forces within the parison as the parison is discharged and the forces cause the parison to significantly change its cross-section shape and possibly cause the parison to close or collapse.
Another type of shaping, often referred to as partial or 2/3 land die shaping or ovalization, changes the thickness of the parison by using the elastic flow component. Flow is more consistent throughout the die/mandrel gap area and parison. Swell, particularly weight swell, is greatest in the shaped areas. Compared to parallel land shaping, the 2/3 land shaping approach is limited in the degree of change possible in the parison wall thickness. Nonetheless, the approach allows control of the discharge to be maintained at all times. Parison collapse does not often occur, and the parison is thicker where the 2/3 shaping is positioned. In 2/3 die shaping, the die is shaped at an angle greater than the land angle. The shaping, however, proceeds over less than the full length of the land.
With both varieties of shaping, the consensus of the industry, in a free drop parison situation, is that the pattern of ovalization must be symmetrical about the die gap in order to balance the forces created by the flow of the polymer material at various points around the die and mandrel discharge orifice.
During die shaping, the depth of the cut, the length of the cut relative to the land, the width of the cut, the shear rate and other specific properties of the plastic resin are all factors which must be considered in shaping the die ring and mandrel for a specific article. The process itself is an art and heavily relies on prior designer/operator experience to achieve satisfactory parisons and articles. If the depth of the cut is too deep relative to the overall die gap, the parison may collapse or close off at its free end as a result of unequal resin flow out of the discharge orifice. If not deep enough, insufficient parison thickness results as portions of the blown article are too thin.
While the above die shaping may have resulted in some success in the ability to extrude parisons having varied wall thicknesses, when blow molding light weight containers, such as one gallon milk jugs, poor parison control has resulted. In light weight containers, an attempt is made to have those portions of the parison which will form the container side walls as thin as possible while ensuring a sufficient amount of material, for container structural reasons, in those portions of the parison which will form the corners of the container. If the thickness variations between the side wall forming portions and the corner forming portions of the parison are too large, the parison will be uncontrolled during extrusion. This is all a function of the depth of the shaping. It is generally known that the maximum depth of the shaping for a lightweight container, such as a milk bottle, which can be put into the die is about 0.0015 to 0.001 inches or about 40% of the die gap. Larger amounts of shaping have been seen to result in localized flow differences which produce an uncontrollable parison drop.
From the above, it can be seen that there still exists a need for a better die shaping technique in order to allow for the blow molding of lighter weight containers.
With the above in mind, it is an object of the present invention to provide a shaped die head that will produce a parison for blow molding light weight containers. An object of the present invention is to provide a shaped die head which will produce a parison with increased thickness wall sections at predetermined locations and which can be controlled during is drop from the extrusion head.
A further object of this invention is to provide a shaped die or mandrel in the die head which allows for larger amounts of ovalization and, therefore, the benefit of thinner side wall sections.
Yet another object of this invention is to provide a shaped die which utilized the visco-elastic properties of the extruded material to manipulate the swell experienced by the plastic material as it exits from the outlet opening of the extrusion head.
In achieving the above and other objects, the present invention provides a shaped die head for use in an extrusion blow molding process. The die head includes a die ring having interior surfaces that define a longitudinally extending bore. Positioned within this bore is a mandrel that, together with the die ring, cooperates to define a flow passageway for the molten plastic material, which can be one of the many plastics, but most commonly HDPE. Both the mandrel and the die ring include frustoconical portions at their outlet ends and these portions cooperate to define the annular outlet orifice or die gap through which the parison is finally extruded.
The invention combines the parallel land and 2/3 land die/mandrel shaping techniques in a single die to allow a greater overall polymer material redistribution in the parison than that possible with each technique individually. All this is done while maintaining control of parison discharge from the extrusion head. This parison material distribution permits a more desirable material distribution in the final blow molded article.
The parallel land shaping cut into the extrusion head tooling essentially controls the degree of polymer material redistribution in the resulting parison. The amount of this shaping, provided as a percentage of the die/mandrel gap, exceeds what normally would be acceptable from a process control perspective. To regain control, 2/3 land shaping is cut into the head tooling circumferentially off-set from the parallel land shaping at a point equally distant from the next parallel land shaping. The 2/3 land shaping does not change the die/mandrel gap in this area, but it does decrease land length which in turn increases parison diameter, weight swell and flow in this area. The localized swell increase slightly reduces the overall shaping effect of the tooling while the localized flow increase greatly improves the overall control of the parison discharge. The net result is a greater degree of shaping while maintaining discharge control.
It should be apparent that the parallel land shaping can be circumferentially cut in one, two, three, four, or more places around the die/mandrel orifice. In the case of one parallel land shaping, one 2/3 land shaping is placed 180 degrees opposite. In the cases of additional parallel land shapings, the off-set 2/3 land shaping is placed between each circumferential pair of parallel land shapings.
While referred to as 2/3 land shapings, it is to be understood that the "2/3" designation is not intended to place a numeric or relational constraint on the length of the second shapings relative to the land length of the frustoconical portion. This designation is only used to show that the length of the second shapings relative to the land length of the frustoconcial portion is less than that of full or parallel land shaping.
According to one embodiment of the present invention, the outlet orifice or die gap is formed with a pair of generally opposed shapings. The shapings extend the full land length of the die and are formed at the same land angle. This parallel land shaping provides for thickened areas in the corner forming portions of the parison. These portions will undergo maximum stretching and expansion during blow molding of the article.
A second set of opposed shapings, herein referred to as flow compensation recesses, are formed in the upstream end of the die ring's frustoconical portion. The flow compensation recesses are circumferentially offset from the first set of shapings by approximately 90.degree. and are formed as partial or 2/3 land shapings.
These second shapings compensate for the increased flow which will subsequently result from the first ovalizations of the die ring. In other words, this second set of recesses increases the flow of molten plastic material in offset areas in anticipation of the increased flow which the first set of ovalizations will subsequently produce. The result is that all of the extruded material, when extruded out through the outlet orifice, flows at substantially the same rate allowing control to be maintained.
The first and second shapings work together to manipulate the swell produced by the visco-elastic properties of the extruded plastic material. In this manner, less swell occurs in the side wall forming portions of the parison and these portions are therefore thinner. The result is a varying wall thickness having a good distribution of the polymeric material about the parison and without the control problems previously seen during the parison drop.
Depending on the actual article configuration, the combination of compensation and ovalizations described above can be provided in alternative embodiments. These alternative embodiments are more fully described below.
As a result of the above combination of shapings and the resulting effect on parison swell, it has been found that larger amounts of shaping than previously seen, above 40% and even approaching 150% of the die gap, are possible. This allows for a reduction in overall container weight by decreasing the side wall thickness without compromising the corner thickness of the container which in turn increases the tensile and loading strength of the container. Furthermore, it has been found that it is now possible to provide a parison having an asymmetrical wall thickness which will still drop in a straight and satisfactory manner out of the parison extrusion head.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.